Overhead travelling cranes and especially bridge cranes of the type powered by electric motors conventionally have a pair of separated parallel horizontal rails with a horizontal overhead beam or bridge member or structure spanning between the separated rails. The bridge is moved horizontally along the rails by one or two electric motors which are controlled together so as to position the beam or bridge at any desired position over its range of travel. A trolley or carriage is mounted to the bridge so as to be moved horizontally along the length of the bridge by a second wound rotor electric motor. A third wound rotor electric motor serves to drive vertically a hook or other hoist assembly which is suspended beneath the trolley. By controlling the three electric motors, the hoist assembly can be placed at any desired position within a three-dimensional volume of space, an article picked up and moved to any other position in the volume.
A brief description of overhead cranes is given at pages 482-483 of Volume 8 of the McGraw-Hill Encyclopedia of Science and Technology (McGraw-Hill Inc., New York, N.Y., 1987). A fuller explanation is to be found in the text Whiting Crane Handbook, by Wm. M. Weaver (4th 1979), published by Whiting Corporation, Harvey, Ill. (the assignee of the present invention).
For electric motor-driven large overhead and bridge cranes, it has been conventional to control the electric motors by means of a manual control. With these such controls, an operator can control the movement of the crane so as to place the hoist at any location within the volume covered by the crane.
The nature of the loads carried and massive components of the crane itself coupled with the inherent characteristics of large size electric motors place restrictions on the manner and timing of the applications of electric power to the crane drive motors in response to the movement of the controls. Thus, in response to a command signal to, e.g., move the beam or bridge from a stopped position at one end of the rails toward another end of the rails, it is conventional to employ a wound rotor electric motor with resistance connected and disconnected out of the motor windings in steps so as to allow the motor to both apply large starting torque to the load and to increase speed without drawing excessive current or risking a danger of "burning out." This has in the past been accomplished by a set of mechanical timers and relays. Thus, when the operator wants to drive the bridge in one direction, the control system responds by coupling voltage through a bank of resistors in series with the motor windings. As time passes and current builds up in the motor windings, relays are activated to shunt out more and more of the series resistance until ultimately the full voltage is applied across the windings of the now up-to-speed rotor. Slowing down and stopping is achieved by the reverse process, with the addition in some circumstances of alternative mechanical brakes applied to the motor.
This prior art arrangement, while generally working well, has several drawbacks or disadvantages. Often, and especially when transporting massive loads, the load and the motor may be subjected to acceleration or deceleration (called "jogging") which can in an extreme case cause harm to the load and system. And, although the system provides a measure of protection against motor burnout, it still requires a skilled operator to prevent excessive wear and tear on the machinery and motors. A "cowboy" operator who slaps the control to full on and then full stop or reverse can still strain the system. Further, such an abusive operator can cause the consumption of excessive electrical energy as well as create greater wear and tear. And with such prior art systems, it is often difficult or impossible, short of failure, to determine the amount of wear and tear on the system. This has lead to a practice of replacing components too early in many cases so as to err on the side of safety, but at increase in the cost of maintenance.
Also, prior control systems have suffered from breakdowns resulting from wear and tear on contactors and corrosion of contact points and breakdown of mechanical timer parts.
Further, a major problem with prior such systems has been the occasional modification of the control parameters by a user's employees. Occasionally, with the best of intentions, an operating engineer will try to "improve" or speed up operation of a bridge hoist or like equipment, for example, by shortening the mechanical timer time-out periods. The result is, often, to overstress the system, create excessive wear, and, occasionally, even result in dangerous system failure.
Thus, there is a need for a control system which decreases the likelihood of "jogging," decreases wear and tear on components, and allows for better monitoring of usage and thus of wear and the need for repair and replacement of components, as well as a means for preventing uninformed changing of operating parameters, and for the monitoring and detecting of abusive operation of the controls.